Geology Reference
In-Depth Information
Table 12.2
Effects of conservation measures on LISEM input
parameters.
Cropland
biological
Fallow
biological
Cropland
mechanical
Orchard ridges
with grass strips
Plant height
1
0.5*
1
1
Plant cover
1.05
1.25
1
2
Leaf area index
1.05
1.25
1
2
Cohesion
1
1
1
1
Added cohesion
1
1.25
1
1.25
Random
roughness
2
1
1.5
1.5
Aggregate
stability
2
1
1
1.25
Manning's
n
0.15*
1.1
1.25
1.25
K
sat
1.25
1.25
1.1
1.25
*These values are not multiplication factors but real values.
Table 12.3
Event characteristics.
980801
980823
990720
990721
000811
000829
Average rainfall (mm)
15.1
13.0
14.1
3.5
11.6
17.8
Max 1-min intensity (mm h
−1
)
a
69.9
47.2
66.2
35.8
49.5
84.9
Time to peak (minutes)
d
15
34
19
32
31
15
Peak discharge (l s
−1
)
5125
701
3589
453
214
8757
Total discharge (m
3
)
3982
b
735
3282
488
199
5893
Total sediment yield (tonnes)
c
1280
96
770
NA
16
2630
a
The given value is a weighted average for the entire catchment. Intensities at individual rain gauges can be much higher (up to
about 120 mm h
−1
for 1-minute intervals in the case of both the 980801 and 990720 events, and up to 190 mm h
−1
for the
000829 event).
b
Hydrograph was incomplete, value is estimated by assuming a linear reservoir.
c
Calculated from total discharge and measured sediment concentration.
d
Time difference between maximum rainfall intensity and peak discharge.
Table 12.3 shows the importance of rainfall
intensity; the lower intensities of the 980823 and
000811 events clearly resulted in much lower
discharges, despite similar total rainfall amounts.
The data suggest that runoff from the catchment
only occurs if more than about 11.5 mm of high-
intensity rain falls. Only the event of 990721 devi-
ated from this trend. This is probably because it
occurred shortly after the 990720 event, so the soil
was still very wet. The 990720 event was special
in that the rainfall amounts in the eastern and
western part of the Danangou catchment were
very different: at the eastern border about 30 mm
of rain fell, while at the western border (about
2 km away) only 3 mm fell. Water samples taken
during events revealed concentrations of up to
500 g l
−1
, which resulted in large total sediment
yields. Table 12.4 shows the LISEM input dataset
as used for the 990720 event. Datasets for the
other events were similar.
